The present invention relates to high pressure arc discharge lamps and is particularly applicable to lamps containing a metal halide fill and employing a tungsten electrode wherein the fill pressure has been increased to such a high value that a starting circuit is required within the lamp.
High pressure metal halide discharge lamps (MHL) are generally comprised of a f used silica or quartz arc tube containing an ionizable fill and having a pair of main thermionic electrodes at the ends. In most applications the electrodes include a relatively high percentage of tungsten. T he electrodes are supported by inleads which include a thin molybdenum ribbon portion extending hermetically through a pinch or press seal in the end of the lamp. Generally, a starter electrode is disposed in the arc tube adjacent one of the main electrodes to facilitate starting. In most lamps, a discharge can be ignited between the starter and the adjacent main electrode at a much lower voltage than between the two main electrodes, and ignition of the arc between the main electrodes is thereby facilitated.
Maintenance of initial lumens in MHL lamps is a problem due to the highly complex chemical reactions occurring in the atmosphere within the arc discharge chamber. More specifically, at the operating temperature of 5,500.degree. K at the center of the arc, to approximately 1,100.degree. K at the wall of the arc tube, which defines a boundary of the plasma, many and various reactions occur. One negative reaction is the transport of metallic and inorganic compounds of tungsten (the main electrode constituent) from the electrode to the walls of the discharge tube during operation of the lamp. The tungsten, in its various compound forms so transported, creates an opaque barrier on the inner wall of the arc tube, thus preventing discharge radiation from being effectively transmitted. In short, significant losses to the level of lumens can occur. This loss of light level from within the discharge is perceived externally as a reduction of light output of the lamp, and thereby reduction in the maintenance of initial lumens. It is believed that the transport of tungsten and tungsten compounds to the walls of the discharge tube occurs through sputtering, evaporation and other chemical mechanisms.
To reduce the transport of tungsten and tungsten compounds to the walls of the discharge tube, a type of MHL lamp has been designed where the cold fill pressure of the inert gas in the arc discharge chamber is increased to a value of at least 50 torr, with the preferred fill pressure being 70 torr and possibly greater than 110 torr. While this design has successfully reduced the transfer of metallic and inorganic compounds, the increased fill pressure, requires a higher than normal starting voltage. Therefore, a voltage multiplier circuit is incorporated into the lamp to assist in the lamp ignition procedure.
The voltage multiplier circuit uses a semiconductor device, such as a diode. Unfortunately, the high temperature creates an environment where metal from the connections is able to diffuse into the semiconductor diode, gradually increasing the diode reverse leakage current. The increase of diode reverse leakage current is destructive and limits the diode usable life.